In the oil and gas exploration and production industry, a wellbore is drilled from surface utilizing a string of tubing carrying a drill bit. Drilling fluid known as drilling ‘mud’ is circulated down through the drill string to the bit, and serves various functions. These include cooling the drill bit and returning drill cuttings to surface along an annulus formed between the drill string and the drilled rock formations. The drill string is typically rotated from surface using a rotary table or top drive on a rig. However, in the case of a deviated well, a downhole motor may be provided in the string of tubing, located above the bit. The motor is driven by the drilling mud circulating through the drill string, to rotate the drill bit.
It is well known that the efficiency of oil and gas well drilling and completion operations can be significantly improved by monitoring various parameters pertinent to the process. For example, information about the location of the borehole is utilized in order to reach desired geographic targets. Additionally, parameters relating to the rock formation can help determine the location of the drilling equipment relative to the local geology, and thus correct positioning of subsequent wellbore-lining tubing. Drilling parameters such as Weight on Bit (WOB) and Torque on Bit (TOB) can also be used to optimize rates of penetration.
In particular, the drilling of a wellbore, preparation of a wellbore for production, and subsequent intervention procedures in a well involve the use of a wide range of different equipment. For example, a drilled wellbore is lined with bore-lining tubing which serves a number of functions, including supporting the drilled rock formations. The bore-lining tubing comprises tubular pipe sections known as casing, which are coupled together end to end to form a casing string. A series of concentric casing strings are provided, and extend from a wellhead to desired depths within the wellbore. Other bore-lining tubing includes a liner, which again comprises tubular pipe sections coupled together end to end. In this instance, however, the liner does not extend back to the wellhead, but is tied-back and sealed to the deepest section of casing in the wellbore. A wide range of ancillary equipment is utilized both in running and locating such bore-lining tubing, and indeed in carrying out other, subsequent downhole procedures. Such includes centralizers for centralizing the bore-lining tubing (and indeed other tubing strings) within the wellbore or another tubular; drift tools which are used to verify an internal diameter of a wellbore or tubular; production tubing which is used to convey wellbore fluids to surface; and strings of interconnected or continuous (coiled) tubing, used to convey a downhole tool into the wellbore for carrying out a particular function. Such downhole tools might include packers, valves, circulation tools and perforation tools, to name but a few.
For a number of years, measurement-whilst-drilling (MWD) has been practiced using a variety of equipment that employs different methods to generate pressure pulses in the mud flowing through the drill string. These pressure pulses are utilized to transmit data relating to parameters that are measured downhole, using suitable sensors, to surface ‘real-time’. Systems exist to generate ‘negative’ pulses and ‘positive’ pulses. Negative pulse systems rely upon diverting a portion of the mud flow through the wall of the drill-pipe, which creates a reduction of pressure that can be detected at surface. Positive pulse systems normally use some form of poppet valve to temporarily restrict flow through the drill-pipe, which creates an increase in pressure that can be detected at surface. The pressure pulses are generated in the flow or supply side of the fluid system.
It will be evident from the above that there is a desire to provide information relating to downhole parameters pertinent to particular downhole procedures or functions, including but not limited to those described above. It is highly desirable to obtain ‘real-time’ feedback on these parameters, so that appropriate adjustments can be made during the operation in question. To this end, there have been proposals to transmit data relating to downhole parameters to surface via fluid pressure pulses. These include but are not limited to those measured in an MWD procedure. One apparatus suitable for this purpose is disclosed in the applicant's International Patent Publication No. WO-2011/004180. The apparatus incorporates a pulse generating device in a wall of a housing of the apparatus, so that a main bore of the housing is not impeded and remains open for the unrestricted passage of fluid, tubing or tools therethrough.
However, problems have been encountered in transmitting fluid pressure pulses to surface, particularly in larger diameter tubing, the pulses being of insufficient magnitude and so difficult to detect at surface. Problems have also been encountered where there are discontinuities in the inner bore diameter of various sections of the tubing (i.e. step changes in diameter). Problems have also been encountered in deep wells, due to signal attenuation. As a result, the data transmitted via the pulses can become lost. The present invention seeks to address these problems.